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Image resolution question...
Old 02-18-2005, 09:28 PM   #1 (permalink)
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Another photographer and I have a bit of a friendly dispute going.

I try to explain my understanding and he looks at me like a cow chewing cud. I can't figure out whether he looks at me that way because I am so stupid, or because I am so hard to understand, or what (maybe I'm both)

He tells me his understanding, and I give him the same look back. I would never claim he is stupid.

I am going to try and explain the "dispute" and would like some good discussion of this.

This is not about who is right or wrong because I freely admit I may be entirely wrong.

Here goes:

Lets say I shoot a JPEG image that is 3008 x1960 pixels (what a D1x shoots)

If I open it in Photoshop, I can zoom in or out.

Zooming in or out does not effect the resolution or change the file size.

If I go to IMAGE-IMAGE SIZE, and change the resolution of the image from 300 DPI to 72 DPI and (and this is very important) I turn OFF the "constrain proportions" and the "resample" box, the image dimensions gets much bigger. So a 3008 x 1960 pixel image at 300 dpi is roughly a 10x7.

If I change the "resolution" to 72 dpi, and turn off the "constrain proportions" and the "resample" boxes, then the image grows to approximately 41x27.

It is my understanding that by doing so, I am not changing the file size of the image, I am not interpolating the image, and I am not losing any data. My image retains the original number of pixels, 3008x1960, it just that if you bunch them up, you image gets smaller and if you spread them out, it gets bigger.

Essentially, to me and my understanding, so long as I am not resampling the image, I am not losing, gaining or interpolating information, I am simply changing the way the original pixels are viewed.....sort of like simply zooming in or out.

My buddy asserts that changing the "resolution" from 300 to 72 causes a loss of data.

NOW, I clearly understand that if you try to "resample" a 72 dpi image to 300 dpi, that photoshop is interpolating, the file size will get bigger too. I understand if I go from a 300 dpi 8x10 to a 72 dpi 8x10 I am loosing a lot of date, file size, etc.

So don't get lost on that.

I also understand as you repeatedly save a jpeg, it will compress and compress, I am not talking about that.

I think if I have a 900 x 300 pixel image I can display it at

100 dpi and it will be a 9x3 image.

At 300 dpi, it will be a 3x1

at 900 dpi, it will be 1 x .3

But the same pixels make up the image, I am neither adding or subtracting "pixels" I am just bunching them up or spreading them out.

(again, I said we have turned the "resample" and "constrain proportions" OFF.

Is this a correct understanding, or am I confused.

Mark
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Re: Image resolution question...
Old 02-18-2005, 09:54 PM   #2 (permalink)
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Your understanding is correct, insofar as I know. The dpi (dots per inch) is a property of the physical [output] device. It does not influence the amount of information in the image, which is determined by the number of pixels in the image file (and the depth of color information in each pixel).

Seems to me that a lot of the confusion is due to trying to think in dpi, instead of actual pixels.
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Re: Image resolution question...
Old 02-18-2005, 11:29 PM   #3 (permalink)
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You're correct. If you don't resample you're not really changing anything except the PRINT SIZE.

And here's another little tidbit. When resizing for the web, dpi doesn't enter into it at all. A 400x600 pixel image is still 400x600 whether you use 72 or 300 dpi. No difference at all.

Paul
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Caveat. . .
Old 02-18-2005, 11:41 PM   #4 (permalink)
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As long as you don't resave the changed image you are essentially correct. However if you should resave as a Jpeg there might be a problem since you have changed the image albeit structurally rather than literally. The resave to Jpeg will lose data.
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Re: Image resolution question...
Old 02-18-2005, 11:42 PM   #5 (permalink)
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I would say that the only caveat is that if the image is a JPG to start and you save as a JPG I would guess that you will lose a bit of data due to the compression. Any modification to a JPG forces the image to recompress but I could be wrong (it wouldn't be the first time)


gary
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Re: Image resolution question...
Old 02-19-2005, 07:52 AM   #6 (permalink)
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You're right.

Does your friend also change all the fonts in his Word documents to 4pt so the file will be smaller? [img]/ubbthreads/images/graemlins/grin.gif[/img]
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You had it 100% right
Old 02-19-2005, 10:49 AM   #7 (permalink)
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Bingo. Six points. Now for the conversion?? LOL
 
 
Two separate things...
Old 02-20-2005, 10:06 AM   #8 (permalink)
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1) When using the Image Size dialog, if "Resample Image" is unchecked, you simply will not lose or gain any information by changing the DPI of the image, as other posters have pointed out. You're right, he's wrong.

2) Why do you keep mentioning that "Constrain Proportions" is off? That's bad. Unless you're deliberately distorting the image for artistic purposes, you should never, ever turn that off, as it will allow you to mess up the picture in various subtle and not-so-subtle ways. If "Resample Image" is off, the setting for "Constrain Proportions" is irrelevant: you can't change the proportions without resampling. (I just tried it: if "Resample Image" is off, you can't change the relative dimensions of the picture regardless of whether "Constrain Proportions" is on or off.) But if you fool with it you might leave it off when it oughtn't be off. If all you do is change PPI or DPI, you won't change the proportions, but if you change the dimensional size in pixels or percentage, you could.

M
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Re: Two separate things...
Old 02-20-2005, 11:19 AM   #9 (permalink)
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Marc:

I guess for this example, I was thinking in terms of turning off any "auto" features most importantly the "resample" box..

The only thing contstrain proportions does is auto fill the other dimension, if you are manually inserting width or height.

So yes, you are right, that aspect is really irrelevant to the discussion.

I also noticed that when you create a "new" document workspace that "resolution" is not even asked, and I kept arguing with my bud that if "resolution" was an important factor you would have to create it in the first instance.

Plus I tried to show him that if you create a blank canvas of a given image and save it as a 300 dpi image or a 72 dpi image the files sizes are the same.

Maybe he read it somewhere or is mixing thoughts about output device options where dpi clearly effect output size.

Mark

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